As circuit size decreases in many mobile communications devices, and associated plastics housings and the like reduce in size, mobile radio handsets are becoming of ever decreasing size. One item of a radio communications device which cannot easily be reduced in size is the antenna. Typically the antenna is one half or one quarter of a wavelength in length along at least one axis and as such cannot easily be reduced. Several variants of antennas of a reduced size, however, have been produced.
One type of a low-profile antenna is the Inverted-L Antenna (ILA), as shown in FIG. 1. The ILA consists of a short monopole as a vertical element and a wire horizontal element attached at the end of the monopole. The height of the vertical element is usually constrained to a fraction of the wavelength. The horizontal element is not necessarily very short, and the total length (horizontal component and vertical component) usually has a length of about a quarter wavelength. For applications such as in GSM handsets, this still means that the antennas is long. A longer length is desirable as it increases antenna efficiency.
The ILA has an inherently low impedance, since the antenna is essentially a vertical short monopole loaded with a long horizontal wire at the end of the monopole. The input impedance is nearly equal to that of the short monopole plus the reactance of the horizontal wire closely placed to the ground plane. A simple and typical modification of an ILA is an Inverted-F Antenna (IFA), as shown in FIG. 2. A small Inverted-L element is attached at the end of the vertical element of an ILA and the appearance is that of a letter F facing the ground plane. This modification can allow the input impedance of an IFA to have an appropriate value to match the load impedance, without using any additional circuit between the antenna and the load.
One drawback of an ILA/IFA consisting of thin wires is the narrow bandwidth, which is typically one per cent or less of the centre frequency. To widen the bandwidth, a modification can be made by replacing the wire element by a plate or by reducing the size of the ground plane, on which the antenna is mounted.
One of the applications of Inverted-L Antennas (ILAs) with respect to portable equipment involves the placement of an antenna element on the top side of a rectangular conducting box. When the conducting box is small compared with the wavelength, the box should be included in the antenna system, since radiation currents will flow on the surface of the box and cannot be ignored.
In the last ten years there has been an world-wide explosion in standards for the radio telecommunication industry covering both cellular mobile telephony and cordless telephone products. This has led to a large number of frequency bands being in use for different systems in different countries and for the requirement for a variety of different handset units to be produced to cover for each radio transmission possibility. Whilst the performance of inverted UF antennas is good in many applications the design is too large to be placed conveniently in small apparatus such as handsets.
Further, it is advantageous to be able to use the same handset for a variety of different radio systems and to be able to switch between them. In addition to the added complexity of the handset electronics, this means the antenna arrangement has to be able to work with a variety of different frequencies and bandwidth requirements. A number of alternatives are possible for the development of dual band handset antennas have been considered. A dual band matching circuit with one antenna can be overly complex and performance can be limited. It is preferred that such dual band handsets employ two antennas, one for each frequency band. Nevertheless, coupling between adjacent antennas can then occur: the antennas need to be sufficiently spaced apart, and thus need to be of small size.
It should also be noted that antennas for personal communication services (PCS) should meet current and proposed legislation/standards for specific absorption rate (SAR).